PARIS Hilton surprised boyfriend Doug Reinhardt by popping out of a cake in a hot pink costume and black diamond mask at his birthday party Saturday at her Beverly Hills mansion. “Paris went all out for her man’s birthday. No expense was spared,” said a source. “He had no idea about the party and was so happy.” Three 6 Mafia and Ya Boy performed for such guests as Kristanna Loken and Hayden Panettiere as home videos of the lovebirds played on screens around the house. The revelry continued until 3 a.m. in the nightclub Hilton had built inside the mansion.